I don't want to enter a debate as for who started the conflict because no one will win. We know how the US has waged illegal wars and supported Jihadists during the Cold War over the years. It is a two way street.
As for the ban, let's call a spade a spade. He announced the ban with much fanfare and media attention. It was almost like a decree and an official announcement. He needs to deliver. A vast amount of his supporters voted for him because they want to see a ban on Muslims entering the US. He cannot just flip flop now. What difference will there be between Trump and a deceiving politician who says one thing during election time and does another afterwards?
To be honest, even he was not sure he will win...So he said so many things which is technically impossible to implement....Rather than paying attention of what he has said earlier, let us wait and see what is he doing now.